A brake apparatus for a vehicle disclosed in JP1997290731A includes a first hydraulic circuit having a first and a second wheel cylinders and a second hydraulic circuit having a third and a fourth wheel cylinders. The brake apparatus for the vehicle disclosed in JP1997290731A regulates braking pressure distribution between the first and the second hydraulic circuits depending on a driving condition of the vehicle.
Specifically, the brake apparatus for the vehicle disclosed in JP1997290731A is provided with a hydraulic pressure pump at one of the first and the second hydraulic circuits for generating auxiliary hydraulic pressure that is added to a master cylinder pressure. When a driver conducts a brake operation while the vehicle is moving in a straight manner, the same level of pressure as the master cylinder pressure is applied to each of the first and the second hydraulic circuits. On the other hand, when the driver conducts the brake operation under a state where a steering angle is large (i.e., vehicle turning state), the hydraulic pressure pump is actuated in order to apply a larger braking hydraulic pressure than the master cylinder pressure to the one of the first and the second hydraulic circuits.
As a result, according to the brake apparatus for the vehicle disclosed in JP1997290731A, a larger braking force is generated when the driver conducts the brake operation while the vehicle turns, comparing to the braking force generated when the driver conducts the brake operation while the vehicle travels straight. However, this means that an increasing characteristic of deceleration of the vehicle with respect to a braking operation variable (i.e., a relationship between the braking operation variable and the vehicle deceleration) differs between a case where the driver conducts the brake operation while the vehicle turns and a case where the driver conducts the brake operation while the vehicle travels in the straight manner.
Generally, it is considered preferable that an increasing characteristic of the vehicle deceleration with respect to the braking operation variable is maintained constant even though the braking hydraulic pressure distribution between the two hydraulic circuits is varied. Further, it may be preferable that the distribution of the braking hydraulic pressure between the two hydraulic circuits is regulated so as to ensure directional stability of the vehicle, in a state where the vehicle is apt to be deflected due to a load condition of the vehicle, the vehicle driving condition and the like.
A need thus exists to provide a brake control apparatus for a vehicle which is not susceptible to the drawback mentioned above.
Additionally, when the driver conducts the brake operation while the vehicle turns, a vertical load applied to each of inner turning wheels is reduced due to a load shift occurring outwardly in a turning radius. Therefore, the brake apparatus for the vehicle disclosed in JP1997290731A may have difficulties in ensuring the directional stability of the vehicle, because cornering force of the inner cornering wheels is unlike to be ensured. In the aforementioned state, it is conceivable that the directional stability of the vehicle is ensured by regulating an increase of the braking hydraulic pressures applied to each of the inner cornering wheels (i.e., an increase of the braking force (longitudinal force)), and preventing a decrease of a limit value of the cornering force (lateral force) generated at the inner cornering wheels.
A need thus exists to provide the brake control apparatus for the vehicle which is not susceptible to the drawback mentioned above.
Furthermore, vehicle behavior differs between a case where the driver conducts the brake operation while the vehicle turns (hereinafter referred to as turn-and-brake operation) and a case where the vehicle turns while the driver conducts the brake operation (hereinafter referred to as brake-and-turn operation). The different vehicle behaviors in the aforementioned cases will be described below with reference to FIG. 51.
Force (frictional force) is generated by a pneumatic tire, which is simply referred to as a tire or a wheel, causing friction against a load surface. More specifically, the tire generates the force (frictional force) by the tire slipping against the road surface. The longitudinal force of the tire is generated by the tire slipping in a tire moving direction (in a longitudinal direction). Additionally, slip of the tire in the tire moving direction is a longitudinal slip and is expressed by slip ratio. The lateral force of the tire (i.e., the cornering force relative to a vehicle body) is generated by the tire slipping in a tire lateral direction. Additionally, slip of the tire in the tire lateral direction is lateral slip and is expressed by a slip angle that is an angle between the tire moving direction and a tire pointing direction.
While the driver conducts a steady turn at a constant speed, the slip angle is generated at front and rear wheels, which results in balancing the cornering force generated at the front wheels and the cornering force generated at the rear wheels. Hence, the sum of cornering force generated at each wheel is balanced against centrifugal force acting on the vehicle. As a result, the vehicle is driven along a turning circle.
When the vehicle speed is reduced by the brake operation while the driver conducts the steady turn at a constant speed (turn-and-brake operation), a vertical load shift from the rear wheels to the front wheels occurs. As a result, the cornering force of the front wheels is increased, and the cornering force of the rear wheels is decreased. An imbalance between the cornering force of the front and the rear wheels generates yawing moment inwardly in the turning direction, which results in the vehicle being moved inward in the turning circle (i.e., occurrence of an oversteering tendency). Referring to FIG. 51, this case corresponds to a shift of a vehicle motional state from a point Yo to a point A by the brake operation.
A case where the vehicle turns while the driver conducts the brake operation (brake-and-turn operation) will be described below. For example, the case where the vehicle turns while a brake control for a translatory moving vehicle is executed corresponds to a shift of the vehicle motional state from a point Xo to a point A by a turning operation. While the longitudinal slip occurs at tires after the brake operation is conducted, the lateral force of the tires is still lower than a case where the brake operation is not conducted, even if the slip angle is generated at the tires. Hence, the cornering force generated by applying the slip angle at the steering wheels by a steering operation is lower than the cornering force generated while the brake operation is not conducted. Therefore, turn-in ability of the vehicle (i.e., response characteristics of the vehicle changing the moving direction) is not sufficient.
As mentioned above, the vehicle behavior occurring when the driver conducts the brake operation while the vehicle turns differs from the vehicle behavior occurring when the vehicle turns while the driver conducts the brake operation. Therefore, stability of the vehicle needs to be enhanced for the turn-and-brake operation. On the other hand, the turn-in ability of the vehicle needs to be enhanced for the brake-and-turn operation.
A need thus exists to provide the brake control apparatus for the vehicle which is not susceptible to the drawback mentioned above.